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Isomerization of Keggin Al13 Ions Followed by Diffusion Rates

Abstract

The solution chemistry of aluminum has long interested scientists due to its relevance to materials chemistry and geochemistry. The dynamic behavior of large aluminum–oxo-hydroxo clusters, specifically [Al13O4(OH)24(H2O)12]7+ (Al13), is the focus of this paper. 27Al NMR, 1H NMR, and 1H DOSY techniques were used to follow the isomerization of the ε-Al13 in the presence of glycine and Ca2+ at 90 °C. Although the conversion of ε-Al13 to new clusters and/or Baker–Figgis–Keggin isomers has been studied previously, new 1H NMR and 1H DOSY analyses provided information about the role of glycine, the ligated intermediates, and the mechanism of isomerization. New 1H NMR data suggest that glycine plays a critical role in the isomerization. Surprisingly, glycine does not bind to Al30 clusters, which were previously proposed as an intermediate in the isomerization. Additionally, a highly symmetric tetrahedral signal (δ=72 ppm) appeared during the isomerization process, which evidence suggests corresponds to the long-sought α-Al13 isomer in solution.

Cover Profile

Invited for the cover of this issue is a group of researchers from the University of California Davis, Colgate–Palmolive Company, and Oregon State University. The image depicts aluminum ions as silver spheres, oxygen atoms as red spheres, carbon as black spheres, and nitrogen as green spheres. Hydrogen atoms were omitted for clarity.

What was the initial goal of this collaborative project?

Our group wanted to understand the role of glycine in the isomerization of the Al13 Keggin clusters. This goal was accomplished using diffusion ordered spectroscopy to monitor the size of the species to which glycine was bound. Glycine not only gave a 1H NMR signal, but some collaborators previously showed that it enhances formation of the rotated-cap isomers.

What is the most significant result of this research?

A new tetrahedral 27Al NMR signal was uncovered that was assigned to the long-sought α-isomer of the Al13 Keggin ion series. Only the ε- and α-isomers can produce such a sharp signal, because it requires high symmetry in the central Al(O)4 site. The intermediate isomers do not have such symmetry. The result matches predictions and is consistent with the mineral form, zunyite.

Who designed the cover and what was the inspiration?

The cover was designed and created by the first author, Dr. Anna F.Oliveri. She has recently begun creating structural images of clusters in solution by using layered tissue paper. Although art has always been a hobby for Anna, she has only recently begun creating table-of-content and cover images.